At the ATP World Finals in London this week, Novak Djokovic took note that Federer's ranking has dropped to No. 6 in the world, that the 32-year-old failed to reach the quarterfinal of any major this year, the first time that has happened since 2004; and that Federer lost in the second round of Wimbledon, a tournament he used to own.

“From my point of view he hasn’t been moving as well this year and I guess that’s one of the reasons he hasn’t had much success," Djokovoic said.

"He is moving maybe slower than he used to, but he is still striking the ball really well and on his day is capable of beating anyone."

Federer has commented on back problems and cut back his schedule. Djokovic beat Federer in three sets in the semifinals of the Paris Masters last week. The pair will play for a second time in four days because they are together in Group B for the London tournament, where only the top eight players make the field. Federer had to fight to the end to make the tournament.